In recent years, requirements for photographic silver halide emulsions have become increasingly severe, and higher levels of photographic performance, including high sensitivity, high contrast, excellent, graininess, and excellent sharpness have been required.
To satisfy such requirements, techniques using tabular grains that intend to improve sensitivity including color sensitization efficiency by sensitizing dyes, and to improve sensitivity/graininess relationship, sharpness, and covering power are disclosed in U.S. Pat. Nos. 4,434,226, 4,414,310, 4,433,048, 4,414,306, and 4,459,353.
The inventors have extensively studied the tabular grains disclosed in the above patents, but have not obtained a fully satisfactory sensitivity/graininess ratio.
This is attributed to the fact that if only the configuration of grains is made tabular, the absorbed amount of the sensitizing dye is increased in proportion to the increase in the surface area/volume ratio, and an increase in sensitivity corresponding to the increase in the amount of absorbed light is not expected. That is, the decrease in the sensitivity due to development restraint, recombination of an electron and a hole, and latent image dispersion caused by the increase in the absorbed amount of the sensitizing dye is great.
In the case where tabular grains are used in a blue-sensitive light-sensitive layer, it is required to increase the amount of absorbed light in the wavelength region inherent to silver halide grains.
To eliminate the above disadvantages and to obtain high sensitivity and high image quality, it is essential to increase the iodide content of emulsions.
The transparency of silver halide emulsion layers is attributed greatly to the effect of light scattering due to the variety of geometrical configuration of grains of emulsion, and in the layer positioned under a layer where the light scattering is high, the sharpness is lowered due to the light scattering in the upper layer.
In Research Disclosure, RD No. 25330 (May 1985), the relationship between the configuration of grains of emulsion and the light scattering is described, and it is to be understood that to improve the sharpness of silver halide photosensitive materials, it is essential to improve the transparency of grains of emulsion.
The light absorption increases in the order of silver chloride, silver bromide, and then silver iodide, and on the other hand the development activity decreases in the stated order, so that it is difficult to secure both light absorption and development activity at the same time.
To overcome this problem, there can be considered the provision of the structure of grains of a silver halide emulsion with a distinct layer structure. That is, the inner core part is made to be a high iodide content phase and the outer shell part is made to be a low iodide content phase, in order to secure both light absorption and development activity.
Japanese Patent Application (OPI) No. 99433/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a method of improving pressure effect (sensitization or desensitization caused by pressure) by using tabular silver halide grains having a grain diameter/grain thickness ratio of 5 or more (i.e., 5/1 or more) and a high iodide content inner phase.
We, the inventors, have studied the above Japanese patent application (OPI) and have not been able to obtain a fully satisfactory result.
In the above Japanese patent application (OPI), since the high iodide content inner phase and the outer phase are formed in the presence of a thioether serving as a silver halide solvent, a distinct layer structure cannot be substantially obtained, and since iodide migrates to the outer phase, the development activity is significantly lowered, to thereby lower the sensitivity.
On the other hand, Japanese Patent Application (OPI) No. 143331/85 discloses a method for producing silver halide emulsions high in iodide content and development activity by making the structure of silver halide grains to be a distinct layer structure having a core part with a high iodide content and a shell part with a low iodide content.
When the above technique is applied to tabular grains having an aspect ratio of 5 or more, the improvement in the sensitivity/graininess ratio has not been fully satisfactory. The inventors have analyzed intensively the cause, and have reached a hypothesis to the effect that when the iodide content deviation among grains of core parts is broad, the grain size distribution after adding the shell is apt to be broad, and the larger the size of the grains, the lower the iodide content becomes.
Based on this hypothesis, the inventors have studied the conditions of the preparation of grains of emulsion and have discovered that when the iodide content deviation among grains is narrow and the size distribution is narrow, the sensitivity/graininess ratio of tabular grains is quite excellent.
In European Patent No. 147,868A, it is disclosed that the sensitivity/graininess ratio can be improved by narrowing the iodide content deviation among grains of a core/shell type emulsion, but it is not disclosed at all that when the iodide content deviation among grains of core parts is broad, the grain size distribution becomes great, and the greater the grain size, the smaller the iodide content becomes. There is no description with respect to tabular grains in that patent. Therefore, it has been quite difficult to expect the effect of the present invention.